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My DW sat in a 3 for the first time yesterday (in fact had a hard time getting her out of the driver's seat!). Previously she had said she didn't want a 3 with black interior. BUT.......yesterday she said the interior isn't black, it is dark grey vegan, almost exactly the same color/material we have in her Prius (the Toyota version of PUP) for the last 4 years. We live in the Palm Springs area where temperatures commonly climb to 117/120 during the summer. She said the Prius interior is fine in those conditions and therefore so will the 3, she is now fine with the 3 interior. As an aside, the Prius also has the ability to pre-cool the interior and she never uses it. YMMV.
There are, literally, hundreds of posts/comments on this. You can do a search either here or in the model S forum. Basically, it comes down to "most people find it isn't an issue".
The roof glass is tinted pretty heavily (at least on the Premium cars) and blocks quite a bit of heat. If you still felt that it was too warm, you could add some 3M Crystalline on top of that for ultimate heat rejection and you'd be totally fine.